Fix scala#10994: align typed pattern syntax to Scala 2

In Scala 2, a typed pattern `p: T` restricts that `p` can only be a
pattern variable.

In Dotty, scala#6919 allows `p` to be any pattern, in order to support
 pattern matching on generic number literals.

This PR aligns the syntax with Scala 2 by stipulating that in a typed
pattern `p: T`, either

- `p` is a pattern variable, or
- `p` is a number literal

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2809,11 +2809,13 @@ object Parsers {
       if (isIdent(nme.raw.BAR)) { in.nextToken(); pattern1(location) :: patternAlts(location) }
       else Nil
 
-    /**  Pattern1     ::= Pattern2 [Ascription]
+    /**  Pattern1     ::= PatVar Ascription
+     *                  | SimpleLiteral Ascription
+     *                  | Pattern2
      */
     def pattern1(location: Location = Location.InPattern): Tree =
       val p = pattern2()
-      if in.isColon then
+      if (isVarPattern(p) || p.isInstanceOf[Number]) && in.isColon then
         in.nextToken()
         ascription(p, location)
       else p

docs/_docs/internals/syntax.md

@@ -318,7 +318,9 @@ TypeCaseClauses   ::=  TypeCaseClause { TypeCaseClause }
 TypeCaseClause    ::=  ‘case’ (InfixType | ‘_’) ‘=>’ Type [semi]
 
 Pattern           ::=  Pattern1 { ‘|’ Pattern1 }                                Alternative(pats)
-Pattern1          ::=  Pattern2 [‘:’ RefinedType]                               Bind(name, Typed(Ident(wildcard), tpe))
+Pattern1          ::=  PatVar ‘:’ RefinedType                                   Bind(name, Typed(Ident(wildcard), tpe))
+                    |  SimpleLiteral ‘:’ RefinedType                            Bind(name, Typed(Ident(wildcard), tpe))
+                    |  Pattern2
 Pattern2          ::=  [id ‘@’] InfixPattern [‘*’]                              Bind(name, pat)
 InfixPattern      ::=  SimplePattern { id [nl] SimplePattern }                  InfixOp(pat, op, pat)
 SimplePattern     ::=  PatVar                                                   Ident(wildcard)

tests/neg/i10994.scala

@@ -0,0 +1,2 @@
+def foo = true match
+  case (b: Boolean): Boolean => ()  // error